Shaft-expanding cone lock

ABSTRACT

A cone piece is tightened into a compatibly shaped hole in the end of a shaft. As the cone piece is tightened, it forces the shaft to expand outwards. As the shaft expands, it creates or increases pressure against a hub, wheel, crank or other mating piece that is positioned on the shaft, locking it into place.

TECHNICAL FIELD

This application relates to a lock for connecting a shaft to a hub,wheel, crank or other mating piece such that torque can be transmittedbetween the two. In particular, this application relates to a lockhaving a frustoconical component that is inserted into the shaft.

BACKGROUND

U.S. Pat. No. 3,957,381 to Schafer discloses a coaxial, double-cone,frictional hub-to-shaft connector. The connector includes two clampingrings. The inner surface of the outer ring and the outer surface of theinner ring are conical surfaces, which engage with each other.Tightening screws to the side of the shaft tighten the connector bydrawing the rings together in axial direction. As the rings are drawntogether, the inner ring clamps against the shaft and the outer ringagainst an inner surface of the hub.

Splines are ridges or teeth on a shaft that mesh with grooves in a hubor gear wheel, for example, that is located on the shaft. Splineconnections allow torque to be transferred between the shaft and themating piece. Some spline connections are prone to backlash. Analternative to splines is a keyway and key, which, however, may not beas durable.

Cranks having a split-ring connecting portion can be tightened around ashaft. The connection may or may not have splines. The connection can betightened using a screw located to the side of the shaft.

A tight press fit can be used to connect a solid shaft to a matingpiece. This requires tight control of tolerances and typically a largeforce to press the two components together.

Other methods of joining a shaft to a mating piece include the use ofthermal expansion and contraction. For example, a shaft is cryogenicallycooled to slip-fit into an interference hole in a wheel hub. As theshaft warms, it expands and forms a strong friction joint with the hub.These joints are difficult to separate.

This background information is provided to reveal information believedby the applicant to be of possible relevance to the present invention.No admission is necessarily intended, nor should be construed, that anyof the preceding information constitutes prior art against the presentinvention.

SUMMARY OF INVENTION

The present invention is directed to a lock for connecting a shaft to ahub, wheel, crank, gear or other mating piece such that torque can betransmitted between the two. The lock includes a frustoconical componentthat is tightened in an axial direction directly into the end of theshaft, forcing the shaft to expand outwards against the mating piece.

The lock does not introduce backlash into the connection, and may have asmaller width, diameter and weight compared to some other types ofconnection. In some embodiments it has the ability to support andtransmit torques from overhung loads and the ability to support axialforces.

Disclosed herein is a shaft-expanding cone lock comprising a cone piecehaving a first frustoconical surface; and a shaft having: an end facedefining an opening of a hole in the shaft; a wall around said hole; asecond frustoconical surface that defines an inner surface of the wall,the second frustoconical surface configured to engage with the firstfrustoconical surface; and a first thread located inwardly from thesecond frustoconical surface and configured to enable tightening of thecone piece into the hole; wherein the wall expands when the cone pieceis tightened into the hole.

In some embodiments, the cone piece comprises a second thread configuredto engage with the first thread. In some embodiments, the cone piece andthe shaft are made from dissimilar materials. In some embodiments, afurther hole passes axially through the cone piece. In some embodiments,the shaft defines an additional threaded axial hole having a diameterless than a diameter of said further hole and accessible through saidfurther hole.

In some embodiments, the shaft-expanding cone lock further comprises amating piece defining a second hole that is dimensioned to receive theshaft at a longitudinal position of the shaft corresponding to thesecond frustoconical surface. In some embodiments, the mating piece andshaft are made from similar materials. In some embodiments, the conepiece is dimensioned to sustain damage before the mating piece when thecone piece is over-tightened.

BRIEF DESCRIPTION OF DRAWINGS

The following drawings illustrate embodiments of the invention, whichshould not be construed as restricting the scope of the invention in anyway.

FIG. 1 is a cross-sectional view of a shaft-expanding cone lockaccording to one embodiment of the present invention.

FIG. 2 is a side view of the cone piece of FIG. 1.

FIG. 3 is a side view of the mating piece of FIG. 1.

FIG. 4 is a side view of the shaft of FIG. 1.

FIG. 5 is an example of a shaft-expanding cone lock according to anotherembodiment of the present invention, incorporated into a biomechanicalenergy harvester.

FIG. 6 is an alternate embodiment in which the connection between theshaft and the mating piece is also splined.

DESCRIPTION A. Glossary

The term “mating piece” refers to a crank, a hub, a wheel, a gear or anyother mechanical component that is to be attached to a shaft, such thattorque can be transferred either from the shaft to the mating piece orfrom the mating piece to the shaft.

The term “cone piece” refers to the component of the shaft-expandingcone lock that is inserted and tightened into the end of a shaft. Thecone piece has at least one frustoconical portion, which engages with acorresponding frustoconical surface inside the end of the shaft.

The term “frustoconical” relates to the shape of a conical frustum, i.e.a cone with its apex removed such that the cut surface is parallel tothe base of the cone.

The term “semi-angle” refers to the angle between the axis of a cone anda generatrix of the cone, the generatrix being a straight line from thecone's apex to the outer edge of the cone's base.

B. Industrial Applicability

The shaft-expanding cone lock is useful for connecting a shaft to amating piece, such that it can readily be removed if desired. It isparticularly useful for connecting mating pieces to shafts in situationswhere space is limited, for example when the mating piece is too narrowto support a viable thread, or when the space available to either sideof the mating piece is too restricted.

C. Exemplary Embodiment

Referring to FIG. 1, an exemplary embodiment is shown of ashaft-expanding cone lock 10, hereinafter referred to a “lock” forbrevity. The lock 10 includes a cone piece 12 and a shaft 14. The lock10 may alternately be considered to include the cone piece 12 and onlythe end portion 15 of the shaft. The cone piece 12 and shaft 14 arecoaxial. In some embodiments the lock 10 may further include a matingpiece 16 or inner portion 17 of the mating piece. The mating piece 16 isreceived on the shaft at a longitudinal position on the shaft thatcorresponds to the location of the inner, frustoconical surface of theshaft. The mating piece 16 has a hole that is coaxial with the conepiece 12 and shaft 14.

The cone piece 12 has a socket 18 for receiving the head of acorresponding driving tool. The cone piece 12 is screwed into the endportion 15 of the shaft 14 to form a threaded connection 20, and thecone piece is tightened by rotating it with the driving tool. As thecone piece 12 is tightened into the shaft 14, the shaft expands outwardsin the region of the frustoconical portion of the shaft wall, which alsocorresponds to the region of the corresponding frustoconical portion ofthe cone piece. As the shaft 14 expands outwards, it presses against thewall of the hole in the mating piece 16, which resists the outwardexpansion of the shaft. The resulting build-up of pressure between theshaft 14 and the mating piece 16 leads to a strong friction fit betweenthe shaft and mating piece, effectively locking them both together. Theresulting friction fit allows the transfer of torque from the shaft 14to the mating piece 16 and vice versa.

Referring to FIG. 2, the cone piece 12 is shown, with frustoconicalsurface 22, thread 24 and inner face 26. The frustoconical surface 22 islocated in the frustoconical portion 28 of the cone piece 12, which isconsidered as the outer portion of the cone piece. The outer portion ofthe cone piece 12 is defined as the portion that faces outwards when thecone piece is inserted into the shaft 14. The thread 24 is located inthe inner portion 30 of the cone piece 12, the inner portion beingdimensioned to screw into and form the threaded connection 20 with theshaft 14. In this particular example, an M8×1.25 thread was used,although other threads are possible.

The angle A represents the angle between the frustoconical surface 22and the axis of the cone piece 12 as viewed from the side, i.e. A is thesemi-angle of the corresponding cone. In this example, angle A is 18°,although other angles are possible.

Referring to FIG. 3, the mating piece 16 is shown. The mating piece 16has a cylindrical hole 40 passing through it from outer face 44 to innerface 46. The wall 48 of the cylindrical hole 40 is dimensioned toreceive the shaft 14. Depending on the embodiment, the diameter of wall48 is dimensioned so that the mating piece 16 is either a slip fit overthe shaft 14 or a press fit over the shaft 14. It is not required forthe press fit to be a tight press fit. If the fit is a press fit, thenas the shaft expands outwards, it presses further against the wall 48 ofthe hole 40 in the mating piece 16. The resulting build-up of additionalpressure between the shaft 14 and the mating piece 16 increases thestrength of the pressure fit between the shaft and mating piece. Themating piece has a thickness T, which in the present, exemplaryembodiment is 3 mm. Other values are of course possible in otherembodiments.

Referring to FIG. 4, the shaft 14 is shown with diameter D. The outer orend face 50 of the shaft 14 has an opening 51 of a hole 52 in the endportion 15 of the shaft. The hole 52 is for receiving and engaging withthe cone piece 12. The hole 52 is coaxial with the axis 53 of the shaft.The hole 52 is surrounded by a side wall 54 that includes afrustoconical surface 55 that corresponds to the frustoconical surface22 of the cone piece 12. Angle A of the frustoconical surface 55 is thesame angle as that of the frustoconical surface 22 of the cone piece 12.The frustoconical surface 55 is located in the outer portion 56 of thehole 52. In particular, the frustoconical surface 22 of the cone piece12 presses against the corresponding frustoconical surface 55 of theshaft 14 as the cone piece is tightened into the shaft. The innerportion 58 of the hole 52, is tapped with a thread 60 that correspondsto the thread 24 on the inner portion 30 of the cone piece 12.

The inner face 64 of the hole 52 is located such that there is a gapbetween the inner face of the hole and the inner face 26 of the conepiece 12 when the cone piece is inserted and tightened into the hole.This is to ensure that the cone piece 12 does not bottom out in the hole52 before the frustoconical surface 22 of the cone piece engages withthe frustoconical surface 55 of the hole. This permits the cone piece 12to be tightened sufficiently to expand the region 66 of the side wall 54in the end portion 15 of the shaft 14, locking the shaft to the matingpiece 16. Likewise, the step 68 (if present, depending on theembodiment) between the frustoconical surface 55 and the thread 60 isdimensioned so as to allow the cone piece 12 to be tightenedsufficiently onto the frustoconical surface 55 before the inward travelof the cone piece is blocked by the step 68.

Region 66 of the side wall 54 of the shaft 14, around the outer portion56 of the hole 52, can be made relatively thin and with a correspondingreduction in weight compared to other connecting techniques in which theshaft needs to be solid in order to support the clamping forces.

The cone piece 12, shaft 14 and mating piece 16 may be made frommaterials such as aluminum, steel, titanium or plastic, for example, orany other engineering material. A low coefficient of friction ispreferable between the cone piece 12 and the shaft 14 so that the conepiece does not bind while it is being torqued. In some embodiments itcan therefore be advantageous to use different materials for the conepiece 12 and the shaft 14 because of the tendency of components of thesame material to stick together or bind, although this is not a hard andfast rule. For example, the cone piece 12 could be made from hardenedsteel and the shaft 14 from titanium. Reduced friction between the conepiece 12 and the shaft 14 means that, for a given cone-tighteningtorque, a greater axial force is generated, resulting in more expansionof region 66 of the shaft wall 54 around the frustoconical portion 28 ofthe cone piece. One exception is that the cone piece 12 and shaft 14could both be made from hardened steel because there would be a lowcoefficient of friction between the two, although it would be moredifficult to use this material.

A high coefficient of friction is preferred between the shaft 14 and themating piece 16. Both the shaft 14 and the mating piece 16 can be madefrom titanium as the coefficient of friction between components bothmade from titanium is high. In other embodiments, it is not necessarythat both the shaft 14 and the mating piece 16 are made from the samematerial. In some embodiments, what normally would be considered a poorsurface finish is allowable for the shaft 14 and mating piece 16interface, because the poor surface finish would provide more frictionthan a polished surface. Also, the intended “poor” surface finish leadsto a lower cost of manufacture.

Furthermore, the frustoconical interface between the cone-piece 12 andthe shaft 14 may be lubricated in order to increase the joint capacity.A lapping compound may optionally be used between the shaft 14 and themating piece 16 to increase friction. Alternately, a thin layer ofanti-size lubricant may be used at the interface between the shaft 14and mating piece 16. However, a dry interface between the shaft 14 andthe mating piece 16 provides improved performance of the lock.

Referring to FIG. 5, portions of a bio-mechanical energy harvester 70employing an exemplary embodiment of the lock 71 are shown. Theharvester 70 is used to generate electrical energy from flexing andextending motion of a knee joint, and optionally to provide power to theknee joint to assist in locomotion. The cone piece 72 is shown insertedinto the shaft 74 and a crank 76 is shown locked onto the shaft by thetightening of the cone piece into the end of the shaft. The lock 71allows all intended loads of the harvester to be transferred between theshaft 74 and the crank 76.

The cone piece 72 includes a hex drive socket 80 for receiving the headof a driving tool. Undercut 82 provides a gap or relief for the threadcutting tool that cuts the thread 84 on the cone piece 72. If desired,cross-sectional area 86 of the cone piece 72, between the undercut 82and the socket 80 can be dimensioned so that the frustoconical portion88 of the cone piece breaks off if the cone piece is over-tightened.

The crank 76 is connected to a link 90 that operates the crank, or thatis operated by the crank. The shaft 74 has a flange 92, which drives aset of gears 94, or which is driven by the set of gears. The shaft 74 ismounted in a housing having sides 96, 97 so that it can rotate. In thisembodiment, a further, threaded hole 98 is drilled in the shaft 74. Thethreaded hole 98 has a diameter less than the diameter of the hole 80for the socket in the cone piece 72. Access to the threaded hole 98 isvia the hole 80 for the socket, the hole used for the socket in thisembodiment passing axially through the cone piece. The threaded hole 98may be used to secure a fastener that prevents the cone piece 72 fromloosening, or for further locking down the cone piece. The hole 98 mayalso be used to secure other components, such as a condyle pad forcushioning the side of the knee.

It can be seen from the example of the harvester 70, which has a compactconfiguration, that there is little available space for connecting thecrank 76 to the shaft 74. Nevertheless, the lock 71 successfully securesthe crank 76 to the shaft.

D. Variations

Smaller values of angle A mean that less axial force is required betweenthe cone piece 12 and shaft 14, and less tightening torque is requiredthan for larger values of angle A. It is expected that angles close to18° would work in a substantially similar fashion, such as 16°, 17°, 19°and 20° or within that range. Modeling carried out on locks 10 withother angles A using finite element analysis has indicated that anglesin the range between 10° and 30° would also work satisfactorily. Valuesfor angle A outside this range would also work, but with lowereffectiveness. Depending on the embodiment, an angle from above 0° toabout 45° would work. However, if A>30°, then the required tighteningtorque may be excessive. If A<10°, then there may a risk of bursting themating piece 16. A further disadvantage of using smaller angles comparedto using larger ones is that greater axial travel of the cone piece 12is required, which necessitates the use of longer parts. Depending onthe dimensions of the application, the room required for the longerparts may not always be available.

The angle A can be selected so that, if the cone piece is over-tightenedin the shaft 14, the frustoconical portion 28 of the cone piece 12breaks off from the inner portion 30 of the cone piece before the matingpiece 16 bursts. In at least one specific example, this is achieved whenthe shaft 14 and mating piece 16 are titanium, the cone piece 12 ishardened steel, the mating piece has a thickness T of 3 mm, the shaft 14has a diameter D of 11 mm, the threaded connection 20 is M8×1.25 andangle A is 18°. Other factors also need to be taken into considerationsuch as the amount and type of lubrication between the frustoconicalsurfaces 22, 55 of respectively the cone piece 12 and the shaft 14.

Other features may be incorporated into the cone piece 12 to ensure thatthe outer portion 28 breaks off if the cone piece is over-tightened. Forexample, there may be an undercut (82, FIG. 4) below the inner end ofthe outer portion 28 and/or there may be an axial hole drilled all theway through the cone piece 12. Both of these result in a reducedcross-sectional area of material in the inner portion of the cone piece12, which allows the outer portion 28 to break away more easily.

For applications where the space is restricted, a fine pitch would bebetter than a course pitch for the threaded joint 20, because it wouldallow for shorter parts and a fine pitch is a little stronger andprovides a little more axial force per unit of torque.

Other means may be used to fasten the cone piece 12 into the shaft 14,such as a screw that is separate from the cone piece and passes throughit to tighten onto the thread 60 in the shaft. In this case, the conepiece would just consist of the outer portion 28, and would not includethe inner portion 30. In other embodiments, a spring could be used toload the cone piece 12.

In normal use, materials, dimensions and threads should be selected sothat the elastic limits of the materials used are not exceeded.

In some embodiments, an extra fastener may be included to prevent thecone piece 12 from becoming unscrewed. It would also be advantageous forthe additional fastener to have a different thread pitch from that ofthe cone piece 12, although this is not absolutely necessary.

The mating piece 16 may be locked onto the shaft 14 so that it is flushwith the end face 50 of the shaft. However, in other embodiments themating piece 16 may be locked onto the shaft 14 either in an overhangingposition or beyond flush.

The thickness T of the mating piece 16 may be equal to, greater than ornarrower than the axial extent of the frustoconical portion 28 of thecone piece 12.

The cone piece 12 may be of unitary construction, or it may be made frommultiple constituent components. The mating piece 16 may be of unitaryconstruction, or it may be made from multiple constituent components.

Another embodiment is shown in FIG. 6 that includes splines 110 on theshaft 114. The cone piece 112 is used to expand the splined shaft 114 toremove any backlash that may be present between the shaft 114 and themating piece 116. The splines 110 are involute, and may have anotherform in other embodiments.

Although the present invention has been illustrated in relation to usein a bio-mechanical energy harvester, it has wide application in respectof other areas, such as in the bicycle industry. In particular, due tothe desire to make bicycles lighter, smaller and lighter components arebeing used. The lock of the present invention, due to its narrowerconstruction compared to other techniques for joining a crank to ashaft, is useful for bicycles in which the crank assembly is to be madenarrower.

It will be clear to one having skill in the art that further variationsto the specific details disclosed herein can be made, resulting in otherembodiments that are within the scope of the invention disclosed.Parameters are given to the nearest decimal place, such that a valuewritten as 16, for example, implies any value in the range of 16±0.5.All parameters, dimensions, materials, proportions and configurationsdescribed herein are examples only and actual values of such depend onthe specific embodiment. Accordingly, the scope of the invention is tobe construed in accordance with the substance defined by the followingclaims.

The invention claimed is:
 1. A shaft-expanding cone lock comprising: acone piece having a first frustoconical surface; a shaft having: an endface defining an opening of a hole in the shaft; a wall around saidhole; a second frustoconical surface that defines an inner surface ofthe wall, the second frustoconical surface configured to engage with thefirst frustoconical surface; and a first thread located inwardly fromthe second frustoconical surface and configured to enable tightening ofthe cone piece into the hole; and a mating piece defining a second holethat is dimensioned to receive the shaft at a longitudinal position ofthe shaft corresponding to the second frustoconical surface; wherein:the wall expands when the cone piece is tightened into the hole; and thecone piece is dimensioned to sustain damage before the mating piece whenthe cone piece is over-tightened.
 2. The shaft-expanding cone lock ofclaim 1, wherein the cone piece comprises a second thread configured toengage with the first thread.
 3. The shaft-expanding cone lock of claim1, wherein the wall expands in a region around the second frustoconicalsurface.
 4. The shaft-expanding cone lock of claim 1, wherein the conepiece defines a socket configured to accept a head of a driving tool. 5.The shaft-expanding cone lock of claim 1, wherein the cone piece and theshaft are made from dissimilar materials.
 6. The shaft-expanding conelock of claim 1, wherein the first and second frustoconical surfaces andthe shaft are coaxial.
 7. The shaft-expanding cone lock of claim 1,wherein the first and second frustoconical surfaces have semi-anglesbetween 10° and 30°.
 8. The shaft-expanding cone lock of claim 7,wherein the first and second frustoconical surfaces have semi-anglesbetween 16° and 20°.
 9. The shaft-expanding cone lock of claim 8,wherein the first and second frustoconical surfaces have semi-angles of18°.
 10. The shaft-expanding cone lock of claim 1, wherein the matingpiece is a crank.
 11. The shaft-expanding cone lock of claim 1, whereinthe second hole is dimensioned to receive the shaft with a slip fit. 12.The shaft-expanding cone lock of claim 11, wherein the wall expands tocreate pressure on an inner surface of the second hole.
 13. Theshaft-expanding cone lock of claim 1, wherein the second hole isdimensioned to receive the shaft with a press fit.
 14. Theshaft-expanding cone lock of claim 13, wherein the wall expands toincrease pressure of the shaft on an inner surface of the second hole.15. The shaft-expanding cone lock of claim 1, wherein the mating pieceand shaft are made from similar materials.
 16. The shaft-expanding conelock of claim 1, wherein the first and second frustoconical surfaces,the shaft and the second hole are coaxial.
 17. The shaft-expanding conelock of claim 1, wherein the mating piece and the shaft form a splinedconnection.
 18. A shaft-expanding cone lock comprising: a cone piecehaving a first frustoconical surface; and a shaft having: an end facedefining an opening of a hole in the shaft; a wall around said hole; asecond frustoconical surface that defines an inner surface of the wall,the second frustoconical surface configured to engage with the firstfrustoconical surface; and a first thread located inwardly from thesecond frustoconical surface and configured to enable tightening of thecone piece into the hole; wherein: the wall expands when the cone pieceis tightened into the hole; a further hole passes axially through thecone piece; and the shaft defines an additional threaded axial holehaving a diameter less than a diameter of said further hole andaccessible through said further hole.
 19. The shaft-expanding cone lockof claim 18, wherein the cone piece is dimensioned to sustain damagebefore the mating piece when the cone piece is over-tightened.